The basic structure of a fuel cell to which the present invention relates, explaining it taking as an example the structure of a general polymer electrolyte membrane fuel cell, is comprised of a proton-conducting electrolyte membrane sandwiched between catalyst layers forming an anode and cathode, further sandwiched at the outside by gas diffusion layers, and furthermore having separators arranged at the outside to thereby form a unit cell. When used as a “fuel cell”, usually a plurality of unit cells are stacked in accordance with the required output. To take out current from a fuel cell (each unit cell) having such a basic structure, gas channels at the separators arranged at the two anode and cathode electrodes supply oxygen or air or another oxidizing gas to the cathode side and hydrogen or another reducing gas to the anode side through the respective gas diffusion layers to the catalyst layers. For example, when utilizing hydrogen gas and oxygen gas, the energy difference (potential difference) between the chemical reaction occurring on the catalyst at the anode [H2→2H++2e−(E0=OV)] and the chemical reaction occurring on the catalyst at the cathode [O2+4H++4e−→2H2O(E0=1.23V)] is utilized to take out current.
Therefore, unless the gas diffusion paths from the gas channels of the separators to the catalysts inside the catalyst layers over which the oxygen gas or hydrogen gas can move, the proton-conducting paths over which protons (H+) generated on the anode catalyst can move through the proton-conducting electrolyte membrane to the catalyst of the cathode, and furthermore the electron-conducting paths over which electrons (e−) generated on the anode catalyst can move through the gas diffusion layer, separator, and external circuits to the cathode catalyst are continuously connected without interruption, current cannot be efficiently taken out.
Inside the catalyst layers, in general, it is important that pores formed at spaces between the materials and forming gas diffusion paths, the electrolyte material forming the proton-conducting paths, and the carbon material or the metal material or other conductive material forming the electron-conducting paths respectively form continuous networks.
Further, for the proton-conducting electrolyte membrane and the proton-conducting paths in the catalyst layers, a polymer electrolyte material comprised of an ion exchange resin such as a perfluorosulfonic acid polymer is used. Such generally used polymer electrolyte materials exhibit high proton conductivity under wet environments and end up dropping in proton conductivity under dry environments. Therefore, to make fuel cells operate efficiently, the polymer electrolyte material must be in a sufficiently wet state. In addition to the gases supplied to the two electrodes, steam must be constantly supplied.
To supply steam, in general the method of running the supplied gases through water warmed in advance to a certain temperature so as to humidify them or the method of directly supplying water warmed to a certain temperature to the cells has been used. A humidifier becomes necessary separate from the cells. However, for the purpose of setting the energy efficiency of the system as a whole high, it is preferable that there be no humidifier consuming energy for warming the water. If there is one, it is preferable that it consume the minimum required amount of energy. Further, for the purpose of making the system as a whole light and smaller, similarly it is preferable that there be no humidifier. If there is one, it is preferable that it be the minimum required size. Therefore, depending on the purpose of use of the fuel cell, sometimes a sufficient humidifier cannot be mounted in the system and the electrolyte material cannot be humidified. Further, even when a humidifier provided with a sufficient humidifying ability for steady state operation is mounted, a low humidified state will unavoidably be fallen into temporarily at the time of startup or at the time of fluctuations in load.
In this way, it is not always possible to use the fuel cell in a wet environment suitable for the electrolyte material, so there are strong demands for fuel cell-use catalyst layers which can exhibit a high performance even under such low humidifying conditions. A high performance fuel cell provided with such catalyst layers and easy to control and operate is therefore desired.
For this reason, in the past, methods have been proposed of using ingredients having hydrophilicity for the gas diffusion layers or catalyst layers or for the intermediate layers arranged between the gas diffusion layers and catalyst layers so as to humidify the electrolyte membrane or the electrolytic materials inside the catalyst layers.
Among these, as a proposal for imparting hydrophilicity to the catalyst layers, Japanese Patent Publication (A) No. 2004-342505 discloses to maintain high cell performance even when reducing the amount of humidification by introducing, for the anode, a catalyst ingredient in the zeolite or titania or other hydrophilic particles or hydrophilic supporting materials.
Japanese Patent Publication (A) No. 2006-59634 discloses a fuel cell exhibiting superior startup characteristics even under a low temperature atmosphere in which the anode catalyst layer has a moisture retention agent comprised of a hydrophilically treated conductive material (hydrophilically treated carbon black etc.) introduced into it.
Japanese Patent Publication (A) No. 2005-174835 discloses, for the purpose of providing a fuel cell able to handle a broad range of humidification conditions, the inclusion of for example hydrophilic particles supporting hydrophobic particles such as “silica particles supporting Teflon® particles” into the catalyst layers.
Japanese Patent Publication (A) No. 2006-155921 proposes a fuel cell using activated carbon as the catalyst supporting material wherein the surface area SBET of the activated carbon by the BET method (Brunauer Emmett Teller specific surface area method) satisfies SBET≧1500 m2/g and the ratio of the surface area Smicro (m2/g) of micropores having a diameter of 2 nm or less to the total pore area Stotal(m2/g) satisfies Smicro/Stotal≧0.5.
Japanese Patent Publication (A) No. 2004-71253 proposes a fuel cell using a supporting material comprised of a carbon material partially including mesoporous carbon particles as the catalyst supporting material.